The various embodiments described herein are directed to low pressure detection of analytes from among sample constituents of a sample in supercritical fluid systems. Examples of supercritical fluid systems include supercritical fluid chromatography (SFC) systems and supercritical fluid extraction (SFE) systems.
Generally, supercritical fluid systems separate (or extract) the sample constituents using a separation unit, such as a chromatographic column. For example, a sample containing various sample constituents, such as chemical compounds and the like, dissolved in a solvent solution may be injected into a fluid stream (mobile phase) with an injection valve, where the mobile phase typically comprises one or more solvents. The sample-containing mobile phase flows through a separation unit, which selectively retains the sample constituents from the sample. In an SFC system, the separation unit comprises a chromatographic column, in which the sample constituents from the sample experience a differential retention with the chromatographic column's stationary phase, e.g., using packing material or sorbent within the chromatographic column, and the relative elution strength of the mobile phase. In an SFE system, the separation unit comprises a chamber or vessel to contain the bulk sample in the presence of an extraction solvent (mobile phase) which extracts the various sample constituents from the sample. The composition of the mobile phase, pressure, and temperature may be varied with time resulting in selective extraction (separation) of sample constituents analogous to chromatographic separations using a column.
The separated sample constituents (or analytes) may then be directed to detectors for detection, collection and/or analysis, where each of the analytes emerges from the separation unit at a different time corresponding to the respective differential retention of that analyte within the separation unit. Detection over time results in “peaks” respectively corresponding to the analytes of the sample, where the magnitude of each peak correlates to the amount of the corresponding analytes in the sample. The detectors include a low pressure detector, such as a mass spectrometer, which receives a relatively small portion of the mobile phase at a relatively low pressure (e.g., atmospheric pressure). The detectors may also include one or more high pressure detectors that receive the bulk of the primary flow stream of the mobile phase, and identify portions of the mobile phase containing the analyte(s) of interest from among the separated sample constituents based on peak detection. These identified portions may be collected by fraction collectors.
As mentioned above, the mobile phase typically comprises a mixture of solvents provided by corresponding pumping systems. In chromatographic systems, the solvents include at least a strong solvent and a weak solvent referring to the solvents relative elution strength in relation to each other and to the stationary phase of the separation unit being used. The strong solvent favors a partitioning of the sample components into the mobile phase, thus lessening retention, or providing faster transiting of the separation unit. The weak solvent favors partitioning of the sample components on the separation unit's stationary phase thus increasing retention, and may serve to moderate the effects of the strong solvent. Attempts are made to balance the mobile phase composition or ratio between the strong and weak solvents in order to provide an acceptable compromise between speed of operation and quality of the analytical results.
SFC systems with packed columns typically use an organic solvent, such as methanol, as the strong solvent and highly compressed dense carbon dioxide (CO2) as the weak solvent. Notably, while the name of the technique, SFC, implies use of fluids in a supercritical state, the actual use includes fluids that while dense, are not necessarily supercritical. Preparative SFC systems typically exist for the express purpose of sample purification. In preparative SFC, the injected sample concentrations often exceed 10 mg/mL. Such high concentrations of sample typically exceed allowable input ranges of many mass spectrometers which operate at or below the 1 μg/ml range. In preparative SFC, the timeliness of the signal from a low pressure detector is important to ensure timely triggering of fraction collectors.
SFE is a process of separating one or more components (extractants) from another (matrix) using fluids similar to a SFC mobile phase as the extracting solvents. Extraction is usually from a solid matrix, but may also be from liquids. SFE may be used as a sample preparation step for analytical purposes, or on a larger scale, to either strip unwanted material from a product or collect a desired product. Again, CO2 is the most used supercritical fluid, sometimes modified by co-solvents such as ethanol or methanol. The properties of a supercritical fluid can be altered by varying the pressure and temperature, allowing selective extraction. A typical SFE system, for example, includes a pumping system for the CO2 and any co-solvents, a pressure cell to contain the sample, the ability to maintain pressure in the system, and a collecting vessel or vessels. The liquid may be pumped to a heating zone, where its temperature may be raised to true supercritical conditions. It then passes into the extraction vessel, where it rapidly diffuses into the solid matrix and dissolves the material to be extracted. The dissolved material is swept from the extraction cell into a separator at lower pressure, and the extracted materials are removed.
In all supercritical fluid systems, the analytes may be detected, as well as analyzed, by a low pressure detector, such as a mass spectrometer. However, the mobile phase output by the separation unit and containing the separated sample constituents is at a pressure and concentration higher than acceptable by most low pressure detectors. Accordingly, there is a need for a timely, efficient, flow splitter providing a controllable means for directing a very small portion of the mobile phase exiting the separation unit to a low pressure detector, without interrupting flow of the portion of the mobile phase remaining at high pressure. There is also a need for the ability to adjust (e.g., in real-time) the low pressure mobile portion of the mobile phase.